The invention relates to a capacitive sensor for measurement of a fuel wall film and, more particularly, to a capacitive sensor for measurement of a fuel wall film in an internal combustion engine.
A capacitive sensor for measurement of a fuel film, comprising at least one condenser element consisting of two electrodes mounted on an insulating support, is known. Both electrodes are also mounted on the top side or one side of the insulating support and a are spaced laterally from each other on the side on which they are mounted. The electrodes of the condenser element or elements are connected with electrical conductors to an analyzer.
A capacitive method for determination of the thickness of a liquid film is known from the ATM Journal (Archives for Engineering Measurement, Section V, 1124-Nov. 19, 1972, pp. 201 to 204 (Archiv fur technisches Messen, Blatt V, 1124-Nov. 19, 1972, Seiten 201 to 204). In this method a capacitive sensor is used, whose electrodes are comb-like and are toothed. These condenser elements form a lateral condenser, whose capacitance changes, when the electrodes and the intervening space are wet with a fluid. Since the capacitance changes because of the wetting by a liquid depend very strongly on the size of the capacitance, several capacitive circuit elements are connected together. It has been shown however that the sensitivity of this kind of sensor is comparatively poor, since on the rear side of the sensor an electric field arises, which provides a parasitic capacitance contribution. This capacitance contribution is larger than the change in the capacitance on the front side of the sensor due to the liquid, because of the dielectric constant of the substrate on which the electrodes are mounted. Thus, despite the larger capacity of the sensor, the analyzable capacitance change, which depends on the liquid film is relatively small. As a result, because of the presence of the interfering electric field the measurement of the wall film can be incorrect.